1. Technical Field
This invention relates to antennas and is especially concerned with drive arrangements for mechanically-steered antennas. The invention is especially, but not exclusively, applicable to antennas for mobile terminals for receiving signal broadcast by satellites and to low cost personal communications terminals requiring steerable, directional antennas.
2. Background Art
In order to maintain adequate reception, it is desirable for a vehicle-mounted antenna to include a directional antenna element or array of elements which can be rotated in azimuth relative to the vehicle so as to track a satellite or other radio signal source as the orientation of the vehicle changes. The antenna element disclosed in U.S. Pat. No. 4,887,091 (Yamada) comprises a reflector which can be folded between open and closed positions. The open position, and hence the elevation angle of the antenna, can be preset using set screws which limit the extent to which the reflector is pivoted when opened. The antenna can be rotated in azimuth by means of a drive motor. A rotary connector in the signal path allows for 360 degrees of rotation. The rotary connector ensures that the signal cable connected between the antenna element and the vehicle will not be damaged by repeated twisting resulting from rotation of the antenna. The connector must be small, low-loss and highly reliable to work in mobile and personal terminals or like high vibration environments. In the event it is used for consumer communications products, it must also be low cost and easy to manufacture. One disadvantage with a stand-alone rotary connector is that its use in antennas which are adjustable in elevation and azimuth makes mechanical construction of the antennas difficult.
Antennas disclosed in French patent No. 1,572,735 and the present applicant's PCT application number WO 94/21002 (or U.S. Ser. No. 08/024,461) permit 360 degrees of rotation without using rotary connectors. In both of these antennas, the antenna element is mounted upon a base member by means of a support which is rotatable in azimuth. The antenna element is connected to the base member by a flexible coupling. Rotation of the antenna element in azimuth is accompanied by rotation of the antenna element about its own boresight axis as a result of torsional forces in the flexible coupling. Although these antennas avoid the use of rotary connectors they do not permit adjustment of elevation angle.
An object of the present invention is to provide for adjustment of the antenna element of such an antenna in elevation as well as in azimuth.